This could be one of a couple things. The first thing I would say you should do is get a worm count. Your horse could have ringworm.
Second check the blanket for fit. Put it on and have someone else walk him around. Watch to see if the blanket moves with your horse’s movement as he walks or if it rubs and makes a swishy sound. If that is the case you need a new blanket.
you should also make sure your blanket isn’t letting water in or trapping sweat, creating a constant wet environment that allows for the grow of nasty bacteria like rain rot. Also make sure your blanket is clean and free of dirt or sweat which could also cause rubs, especially if your horse lives in it 24/7
I think it is most likely a worm and this will be shown by a decal test. A call to your vet may be in order.